This invention relates to a class of alloys acting as moisture and gas getters, and it relates more particularly to a class of alloys having the property of reacting with moisture and gases generated inside enclosed regions at elevated temperatures.
Nuclear reactors are presently being designed, constructed and operated in which the nuclear fuel is contained in fuel elements which may have various geometric shapes, such as plates, tubes, or rods. The fuel material is usually enclosed in a corrosion-resistant, non-reactive, heat conductive container or cladding. The elements are assembled together in a lattice at fixed distances from each other in a coolant flow channel or region forming a fuel assembly, and sufficient fuel assemblies are combined to form the nuclear fission chain reacting assembly or reactor core capable of a self-sustained fission reaction. The core in turn is enclosed within a reactor vessel through which a coolant is passed.
The cladding serves two primary purposes: first, to prevent contact and chemical reactions between the nuclear fuel and either the coolant or moderator if present, or both; and second, to prevent the highly radioactive fission products, some of which are gases, from being released from the fuel into the coolant or moderator or both. Common cladding materials are stainless steel, aluminum and its alloy, zirconium and its alloys, niobium (columbium), certain magnesium alloys, and others. The failure of the cladding, due to the build-up of gas pressure or high temperatures in the fuel, can contaminate the coolant or moderator and the associated systems with intensely radioactive long-lived products to a degree which interferes with plant operation.
Problems have been encountered in the manufacture and in the operation of nuclear fuel elements which employ certain metals and alloys as the clad material due to the reactivity of these materials under certain circumstances. Zirconium and its alloys, under normal circumstances, are excellent materials as a nuclear fuel cladding since they have low neutron absorption cross sections and at temperatures below about 600.degree. F. are extremely stable and non-reactive in the presence of demineralized water or steam which are commonly used as reactor coolants and moderators. Within the confines of a sealed fuel rod, however, the hydrogen gas generated by the slow reaction between the cladding and residual water may build up to levels which under certain conditions can result in localized hydriding of the alloy with concurrent deterioration in the mechanical properties of the alloy. The cladding is also adversely affected by such gases as oxygen, nitrogen, carbon monoxide and carbon dioxide at all reactor operating temperatures.
The zirconium alloy cladding of a nuclear fuel element is exposed to one or more of the gases given above during irradiation in a nuclear reactor in spite of the fact that these gases may not be present in the reactor coolant or moderator, and further may have been excluded as far as possible from the ambient atmosphere during manufacture of the cladding and the fuel element. Sintered refractory and ceramic compositions, such as uranium dioxide and others used as nuclear fuel, release measurable quantities of the aforementioned gases upon heating, such as during fuel element manufacture or especially during irradiation. These gases react with zirconium alloy clad material containing the nuclear fuel. This reaction can result in the embrittlement of the cladding which endangers the integrity of the fuel element. Although water and water vapor may not react directly to produce this result, at high temperatures water vapor does react with zirconium and zirconium alloys to produce hydrogen and this gas further reacts locally with the zirconium and zirconium alloys to cause embrittlement. These undesirable results are exaggerated by the release of these residual gases within the sealed metal-clad fuel element since it increases the internal pressure within the element and thus introduces stresses not anticipated in the original design of the clad tube.
In light of the foregoing, it has been desirable to eliminate water, water vapor and gases reactive with the cladding from the interior of the cladding throughout the time the nuclear fuel is used in the operation of nuclear power plants. One such approach has been to find materials which will chemically react with the water, water vapor and gases to eliminate these from the interior of the cladding, which materials are called getters. While several getters for water and water vapor have been found, such as the zirconium-titanium getter set forth in U.S. Pat. No. 2,926,981, it has remained desirable to develop a getter having equal or even greater rapidity of reaction with moisture and gases, and having the additional feature of producing negligible hydrogen gas during the reaction with moisture.